The present invention relates to an instrument for conducting secondary-ion mass spectrometry (SIMS) and, more particularly, to a direct imaging type SIMS instrument which can make analysis of a sample by time-of-flight (TOF) mass spectrometry.
Secondary-ion mass spectrometry involves bombarding a sample with a beam of primary particle ions and analyzing the secondary ions that emanate from the sample surface. The secondary ions are then introduced into a mass analyzer, where they are mass analyzed. As a result, the composition of a microscopic region on the surface of the solid sample can be elucidated. Instruments for conducting SIMS are broadly classified into two types: scanning type which scans an analyzed region with a sharply focused primary beam to obtain an ion image; and direct imaging type which bombards the whole analyzed region with a primary beam of a relatively large diameter and obtaining an ion image on the principle of an ion microscope.
FIG. 1 shows the ion optics of one example of the direct imaging type SIMS instrument. A primary ion beam I.sub.1 produced from an ion source IS has a relatively large diameter. This beam is caused to impinge on the whole analyzed region on a sample S. Secondary ions I.sub.2 emanating from this region are sent to a mass analyzer MS through a transfer optics TO. In this mass analyzer, only secondary ions having a certain mass are selected and then projected via a projector lens Lp onto a two-dimensional detector such as a fluorescent screen FS. Thus, an ion image is obtained with the certain mass.
In the ion optics shown in FIG. 1, electrostatic lenses L.sub.11 and L.sub.12 are used to form the primary ion beam. The transfer optics TO consists of electrostatic lenses L.sub.21, L.sub.22, L.sub.23. A slit SL.sub.1 is disposed at the entrance to the mass analyzer MS. The ion optics further includes an intermediate lens L.OMEGA., an energy slit SL.sub.2, and a mass-selecting slit SL.sub.3.
In the instrument shown in FIG. 1, the secondary ions emitted from the sample surface have a large energy spread and, therefore, the mass analyzer MS consists of a double-focusing mass analyzer in which a spherical electric field EF and a uniform sector magnetic field MF are connected in tandem. The direct imaging type SIMS instrument as shown in FIG. 1 is disclosed by George Slodzian in his book Applied Charge Particle Optics, 1980, in the third chapter (III. Direct Imaging Instruments), pp. 17-19, of an article entitled "Microanalyzers Using Secondary Ion Emission."
In the aforementioned prior art instrument, it is inevitable that the mass analyzer has a large-scaled structure, because it consists of a series combination of the electric field, the lens L.OMEGA., and the magnetic field. The prior art instrument can only provide mass-filtered ion images in which the contrast is given by the presence or absence of ions of a specified mass.
It is known that irradiation of a primary ion beam can hardly ionize special substances such as gold, because the ionization efficiency widely differs among elements or substances. In so-called laser pulsed ionization, a sample is ionized with a pulsed laser beam of a high intensity. It is known that with this ionization process, almost all substances including gold can be ionized at substantially the same high efficiency.
The secondary ion yield ratio (number of emitted secondary ions/number of sputtered neutral particles) of some kinds of samples is extremely small and only a small quantity of secondary ions can be obtained by irradiation by a primary ion beam. In such case, the produced secondary neutral particles can be ionized by irradiation of a pulsed laser beam, for mass analysis. This method is known as secondary neutral particle mass spectrometry (SNMS). Also in this case, pulsed ions are generated.
Therefore, in order to analyze a sample including special substances, the application of the aforementioned pulsed ionization method is required In the SIMS instruments, the strength of the magnetic field must be changed to analyze ions of different masses. Consequently, it is difficult to apply the pulsed ionization process to the proposed instrument.
It is known that time-of-flight (TOF) mass spectrometry is suited to cases where produced ions are pulsed.